In numerous fields of technology it is required to vapor-deposit very thin layers of material on carriers to obtain given properties. Examples are interference filters and compact-disc plates. In order to be able to vapor-deposit from the gas phase, gas plasmas must previously be generated, i.e. gases with positive and negative charges as well as neutral particles.
For the production of such plasmas numerous methods are known. A currently widely used method consists in generating an electron cyclotron resonance with the aid of microwaves and static magnetic fields whereby a strong ionization of gases becomes possible. Such microwaves must be fed in with hollow waveguides or similar waveguides since they comprise a frequency range of 300 MHz to 300 GHz or a wavelength range of 1 m to 1 mm. In this range electromagnetic waves behave quasi-optically.
However, plasma excitations are also known in which electromagnetic waves in the UHF range or in the range below 100 MHz are used (U.S. Pat. No. 4,691,662; Oechsner, Plasma Physics, Vol. 16, 1974, pp. 835 to 844; Boswell, Plasma Physics and Controlled Fusion, Vol. 26, No. 10, 1984, pp. 1147 to 1162). If these waves impinge on magnetic fields they propagate parallel to them. In geophysics these waves are known as whistler waves. They originate from lightning strokes and run along the magnetic field lines through space. In the region in which these field lines reach the earth surface a whistling sound is heard in a loudspeaker which goes from high to low sounds. In solid state physics the whistler waves are also called helicon waves (Harding and Thonemann, Study of helicon waves in indium, Proc. Phys. Soc. 1965, Vol. 85, pp. 317 to 328).
To generate whistler or helicon waves in the laboratory circularly polarized waves must first be generated, because the whistler and helicon waves are circularly polarized waves. In this connection it is known for example (Boswell, op.cit., FIG. 4; Boswell, Perry, and Emami, Le Vide, Les Couches Minces, Supplement au No. 246, March/April 1989, FIG. 1) to place an 8.8 MHz oscillation from an oscillator and an amplifier onto a coaxial cable which via two variable high voltage capacitors in a pi-network is adapted to an antenna. Of disadvantage herein is the fact that the special geometry of the antenna makes a matching in terms of power of the waves to the plasma difficult and that the ignition behaviour is problematic.
A microwave plasma generator is also known which includes mutually perpendicular electrodes arrayed around a container (U.S. Pat. No. 4,792,732). In this design a first pair of electrodes is connected through a 90.degree. phase shifter to a voltage supply, and a second pair of electrodes is connected directly to this supply. The electrodes generate a circularly polarized field in the container. The energy is supplied longitudinally through the entire chamber. Only cross-sectional dependence is considered in the applied fields, and homogeneity is assumed in the longitudinal direction of the container. Helicon waves cannot be excited by this device because such excitation must be significantly shorter than the wavelength. Because the known plasma generator operates with microwaves of very short wavelengths, this condition is not met.
In the case of other known arrangements with which circularly polarized waves can be generated due to the antenna geometry it is not possible to carry out a matching in terms of power in a simple manner (U.S. Pat. No. 4,160,978).